FSI combines hundreds of pages of print materials with hundreds of verbal audio drills. I used it to supplement a French course in college and my professor asked me how I advanced so quickly. The course materials were doing very little for the rest of the class because they didn't force natural language thinking. FSI grammar drills make you think on your feet in conversations with different roles and grammatical rules.

I had also tried Rosetta and Pimsleur at the time. Years later I tried Duolingo for another language. To me they all feel like fun toys, but only toys compared to FSI. If I wanted to learn another language, I'd check if it had an FSI course.